Enceladus presents a puzzle for Cassini team

Enceladus, a tiny moon orbiting Saturn, appears to be venting water into space from a series of fractures over its south pole.

The water vapour is pouring out of the body through an anomalous hotspot centred over its south pole, in only the second extra-terrestrial observation of thermal activity in the solar system.

According to the latest data from Cassini, the moon might have lost as much as five per cent of its mass since its formation, and figures suggest it is losing enough material to account for Saturn's E-ring.

The difficulty is, scientists can't explain how the south pole region might be getting so hot.

All the instruments on board Cassini point to something very unusual happening at Enceladus' south pole, the area towards the bottom of the moon in the image here, marked by four heavy crevasses, known as the 'tiger stripes'.

Speaking at a press conference in London yesterday, the researchers professed themselves "happy to be baffled" by the tiny moon. "If all you're doing is confirming your colleagues papers, it is no fun," said Torrence V. Johnson of NASA's Jet Propulsion Laboratory.

He explained that the two known sources of heat: radiogenic heating and tidal heating, are not enough, either alone or in combination to account for the behaviour. "Also, you would expect tidal heating to affect the whole body, or at least both poles. Its possible there is something special about Enceladus' structure that is focusing the tidal heating all in one area," he said.

Another possibility is that Enceladus was in a more eccentric orbit some time ago, and that this orbit provided enough tidal heating to kickstart the process we see the results of today. The surface of Enceladus's south pole is very young, so this could have been a relatively recent event.

Johnson says that the researchers have developed several 'cartoon sketches' of what might be causing the heating in the region, but stressed that he would not refer to their ideas as fully-formed hypotheses.

The first sign that something odd was going on was spotted by Professor Michele Dougherty. While reviewing magnetometer data, she noticed that Saturn's magnetic field lines were being deflected by the moon to a greater degree than its size could account for. She suspected the body might have an atmosphere - which would be unusual for a body just 500km in diameter, and would have to have an internal source - and petitioned mission planners for a closer fly by to see what was going on.

So, on 14 July this year, Cassini flew within 176km of the surface of the moon, and returned some spectacular images.

The data it gathered during that fly-by revealed that the moon does indeed have an atmosphere, and a very strange one at that. "It is probably around the entire body, but seems to be concentrated at the south pole," Dougherty explains, comparing the plume of material emanating from the moon to a cometary jet.

Temperature data from the surface revealed that the south pole is conspicuously hot, in contrast to the researchers' expectations.

John Spencer from the Southwest Research Institute in Colorado, analysed the data from the Composite IR Spectrometer said that they had expected the hottest temperatures - no more than 80 Kelvin - to be found near the equator, at noon when the sun was shining directly overhead.

What they actually found was a hotspot of around 85 Kelvin over the south pole. Zooming in for a closer look revealed that the temperature directly over one of the crevasses was 91 Kelvin: "Distinctly warmer than its surroundings," Spencer confirms.

Data from the visual IR mapping spectrometer provided more evidence of some kind of internal heat source.

The ice crystals near the tiger stripes are highly crystalline, as compared to the less ordered ice covering the rest of the moon. Bob Brown from the University of Arizona says that this forces the conclusion that the surface in that area is either very hot, or very young, or both.

The researchers also found simple chains of carbon on the surface near the fractures. "There is something very special about the fractures," Brown said.

Cassini has two more flybys of Enceladus scheduled, and the researchers will be keen to get more data from those to help them unravel the mystery. ®